1. Field of the Invention
The invention is directed to a plasma treatment apparatus.
2. Description of Related Art
In plasma treatment of workpieces, a glow discharge in a vacuum is generated between the workpiece and a counterelectrode, while certain gases for plasma generation are present at a very small pressure. In this process, gas atoms interact at high temperatures with the surface of the workpiece for changing the properties of the surface in the desired manner. For instance, there are known methods for plasma carburizing, plasma nitriding or plasma coating of metals. The workpieces to be treated are placed in an oven while still being cold. The workpieces are cleaned by a treatment performed at low pressure, are heated at increasing pressure, then are subjected to plasma treatment and are finally cooled off.
A plasma treatment apparatus as described in U.S. Pat. No. 4,645,981 comprises a vacuum container wherein the workpiece is to be placed. The workpiece is connected to an electrode, and the inner wall of the vacuum container or the container itself is used as a counterelectrode. In this known apparatus, and also in other vacuum containers for glow-discharge treatment of workpieces, continuous operation is not possible. Instead, the workpieces must be placed in the treatment chamber by batches, then undergo treatment and subsequently be removed again. No possibility exists for treating a plurality of workpieces in such a manner that they are subjected to different lengths or types of treatment. After termination of a treatment, for exchanging the workpiece, the vacuum has to be relieved while also the gas and the heat contained in the vacuum chamber escape. Continuous operation of the vacuum chamber for glow-discharge treatment is impossible because the occuring high temperatures in the range of 500.degree. C. to 2000.degree. C. do not allow the use of conveying means. Due to the high temperatures and the discharge conditions in the vacuum chamber, those components that contact each other tend to undesired welding and erosion. Therefore, in known plasma heat treatment apparatuses, all of the components are stationary during treatment.
A treatment apparatus known from U.S. Pat. No. 4,501,766 has a treatment chamber wherein a rotating means is arranged in the manner of a carousel. Said rotating means, in turn, holds a plurality of rotatable workpiece supports onto which the workpieces are set. Upon rotation of the rotating apparatus, a gear drive serves for rotating the individual tool supports on the rotating means. The treatment apparatus is a film-depositing apparatus working with high-frequency energy wherein the workpieces are heated and connected to an electric potential while a channel provided for conveying the workpieces therethrough is connected to a counterelectrode. High-frequency treatment apparatuses of this type work at a relatively low temperature so that operation of the rotating means and rotation of the workpiece carriers within the chamber are easily performed as far as the rotating means is concerned.
Different from treatments carried out at relatively low operating temperatures, plasma diffusion processes require high temperatures in the range of 500.degree. C. to 2000.degree. C.. Normally, in this case, plasma treatment is performed by pulsed direct current discharge in a vacuum chamber. In contrast to coating processes, plasma diffusion processes, due to the temperature and vacuum conditions, allow no use of parts coming into direct contact with each other. Such componets would undesirably be welded to each other and be destroyed.
It is an object of the invention to provide a plasma treatment apparatus which is adapted for performing different treatments, particularly with different lengths of treatment and different temperatures of the workpieces, in the same treatment chamber, and which is adapted for treatments requiring relatively high temperatures.